Microencapsulation of various core materials in polymers by nonreactive processes such as solvent evaporation and phase separation is known. See, for example, U.S. Pat. Nos. 3,657,144 to Yoshida, 3,732,172 to Herbig et al., 3,523,906 to Vrancken et al., 3,960,757 to Morishita, et al., and 3,660,304 to Matsukawa. The Yoshida '144 patent discloses a process for encapsulating discrete solid particles or droplets of aqueous liquids and the Herbig et al. '172 patent also discloses encapsulation of solid particles. The Vrancken et al. '906 patent discloses encapsulation of water droplets or compounds dissolved or dispersed in water, while the Morishita, et al. '757 patent discloses bioerodible or sustained release microcapsules of medicaments. The Matsukawa '304 patent discloses encapsulation of oily liquids in thermosetting resin-type polymers, designed for the release of dyes upon fracturing of the polymer shell, the encapsulation being accomplished by mixing a solution of a low boiling point solvent and a hydrophobic polymer with a high boiling point solvent, emulsifying and evaporating the low boiling point solvent. These processes are primarily physical and do not involve chemical reactions, thus allowing the use of more varied polymeric materials for the microcapsule walls, avoiding the necessity of a separate crosslinking or strengthening step, and enabling the encapsulation of labile or reactive ingredients that cannot be encapsulated via processes such as interfacial polymerization.
Controlled release of biologically active microencapsulated ingredients is also known. See, for example, U.S. Pat. Nos. 4,316,884, 4,172,119, 4,056,610 and 3,977,922, as well as J. Chem. Ecol. 7 (1981) 867, and Scher, "Controlled Release Pesticides," 53 ACS Symposium Series, Chapter 12 (1977). However, the duration of biological activity disclosed is generally limited to a period of from a few days to a month.
There is, therefore, a need for a controlled release microcapsule capable of providing sustained release of biologically active core ingredients at a substantially constant rate over an extended period of time through a strong, impact-resistant, yet permeable polymeric shell. Such strong, impact-resistant, and permeable microcapsules would be extremely useful for agricultural applications where the microcapsules must be applied by spraying with pumps and nozzles, which subjects the microcapsules to high shear forces, and where long-term delivery, i.e., up to six months and more, of the biologically active ingredient is desirable. Such microcapsules would also be useful for delivery of pharmaceuticals, flavors, sweeteners, and the like in chewable formulations where again the microcapsules are subjected to high forces and fracture is to be avoided.
It is therefore a principal object of this invention to provide novel microcapsules for the sustained and controlled release of biologically active core ingredients through a durable, impact-resistant, permeable polymeric shell.